1. Field of the Invention
The invention relates to a method for cooling a fuel injection system and to a fuel injection system for an internal combustion engine.
2. Description of the Prior Art
Injection systems of internal combustion engines, especially if they are operated with Otto fuel tend to form vapor bubbles under certain conditions. For instance, it can happen that the high-pressure fuel pump and/or the common rail, after the shutoff of the engine, heat up so much that the boiling temperature of the fuel is reached or exceeded. The consequence is vapor bubbles in the fuel injection system, which in a so-called xe2x80x9chot startxe2x80x9d of the engine that occurs soon after shutoff impair the starting performance of the engine.
From German Patent Disclosure DE 195 39 885 A1, it is known, in order to improve the hot-starting performance of an internal combustion engine, to flush the fuel injection system briefly with relatively cool fuel upon starting and then to raise the pressure in the fuel feed line.
The object of the invention is to furnish a method for cooling a fuel injection system and to furnish a fuel injection system, which further improve the hot-starting performance of the engine.
According to the invention, this object is attained by a method for cooling a fuel injection system for internal combustion engines, having a prefeed pump that pumps fuel from a tank via a feed line into a high-pressure region, the high-pressure region including a high-pressure pump, having a return line for removing excess fuel from the high-pressure region, and having a connecting line, connecting the feed line and return line, the flow through the connecting line being controllable by means of a scavenging valve, in which the scavenging valve is closed when upon turn-on of the engine a turn-on criterion has been reached, at least parts of the high-pressure region are scavenged with fuel from the fuel tank, and the scavenging valve is opened as soon as a turn-off criterion is reached.
In this method of the invention, the pressure elevation and the scavenging process begin simultaneously, so that the removal of any vapor bubbles that may be present from the high-pressure region and the compression of these bubbles occur simultaneously. The result is improved hot-starting performance of the engine.
Variants of the invention provide that a characteristic temperature is used as the turn-on criterion, in particular the temperature of the fuel in the high-pressure region and/or the temperature of components of the fuel injection system and/or the temperature of the air in the surroundings of the fuel injection system. This characteristic temperature can be measured or calculated by means of model calculation. By using a characteristic temperature as the turn-on criterion, it is assured that whenever there is a risk of vapor bubble development, a scavenging operation and compression of the vapor bubbles by an elevation of pressure in the fuel injection system is tripped. On the other hand, unnecessary pressure elevations in the feed line and in the high-pressure region upon engine starting are avoided.
Further features of the method of the invention provide that the scavenging valve is opened in time-controlled fashion, and/or that the scavenging valve is opened as a function of the course over time of the characteristic temperature, so that an unnecessary load on the prefeed pump is avoided and nevertheless, the development of vapor bubbles is suppressed immediately after the hot start of the engine.
Thus by way of example, the scavenging valve can be opened if the characteristic temperature has undershot a predetermined value.
It is also possible for the time during which the scavenging valve remains closed to be defined as a function of the characteristic temperature, and/or for the scavenging valve to be opened as a function of the fuel quantity pumped by the prefeed pump for scavenging purposes, or for the scavenging valve to be opened as a function of the air flow rate aspirated by the engine. In these variants of the method of the invention as well, effective removal and effective compression of the vapor bubbles from the high-pressure region are assured, without overloading the prefeed pump. In addition, a temperature sensor for ascertaining the turn-on criterion can be dispensed with.
The hot-starting performance of an internal combustion engine can be further improved if the scavenging valve is closed as soon as voltage is applied to the terminal of the vehicle. As a result, the pressure elevation occurs at the earliest possible instant, which further improves the hot-starting performance.
In a further feature of the invention, the turn-on criterion is reduced after the fuel tank has been filled, since xe2x80x9cfreshxe2x80x9d fuel has a lower boiling point than fuel that has been in the tank for a relatively long time.
The object stated at the outset is also attained by a fuel injection system for internal combustion engines, having a control unit, having a prefeed pump that pumps fuel from a tank via a feed line into a high-pressure region, the high-pressure region including at least one high-pressure pump, having a return line for removing fuel from the high-pressure region, the return line having a relief throttle and a low-pressure regulator, in which between the feed line and the return line, a connecting line with a scavenging valve and a mixing line with a check valve are provided; the mixing line discharges into the return line upstream of the connecting line; and the connecting line discharges into the return line upstream of the low-pressure regulator.
This fuel injection system of the invention is very simple in design, since to improve the hot-starting performance, only one connecting line with a scavenging valve and one mixing line with a check valve have to be provided. The pressure control valve presents a flow resistance upon starting of the engine, so that a pressure elevation in the feed line and the high-pressure region ensues as soon as the prefeed pump is in operation and the scavenging valve has closed. To improve the hot-starting performance, all that is needed is to monitor a turn-on criterion and to close the scavenging valve if necessary. Because of the simultaneity of the scavenging and compression of the vapor bubbles, the engine turns over after only a very short time even in a hot start.
In a variant of the invention, a relief throttle is provided between the discharge point of the mixing line and connecting line into the return line, and that the check valve is loaded by a spring. The opening pressure xcex94p of the check valve, which can also be embodied as a spring-loaded check valve, causes a pressure difference to occur at the relief throttle as soon as the scavenging valve is closed. By a suitable adaptation of the opening pressure xcex94p of the check valve, the pumping capacity of the prefeed pump, and the opening pressure xcex94p of the check valve, the scavenging flow in the high-pressure region of the fuel injection system can be adjusted. Because the check valve operates as a function of pressure and need not be triggered by the control unit, the effort: and expense for laying a signal line is omitted, and there is less of a load on the control unit. The control unit can be relieved still further if the scavenging valve is opened when without current.
Further features of the invention provide that the check valve is spring-loaded, so that the opening pressure xcex94p of the check valve is easily adjustable.
In another variant of the invention, the scavenging valve is open when without current, so that the scavenging valve has to be triggered only in the case of a hot start of the engine.
The above-stated object is also attained by a fuel injection system for internal combustion engines, having a control unit, having a prefeed pump that pumps fuel from a tank via a feed line into a high-pressure region, the high-pressure region including at least one high-pressure pump, having a return line for removing fuel from the high-pressure region, wherein the return line has a check valve and discharges into the feed line, and having a leakage line for removing fuel from the high-pressure region into the tank, in which a check valve is provided in the return line, and a low-pressure regulator is provided in the leakage line.
In this fuel injection system of the invention, the high-pressure pump is permanently cooled, so that both during a hot start and in the heating mode, the formation of vapor bubbles is effectively suppressed. By a suitable adaptation of the pumping capacity of the prefeed pump and the pressure established by the low-pressure regulator in the leakage line, the scavenging quantity can be adjusted such that on the one hand the tank is not unnecessarily heated up, and on the other a more-reliable hot start and hot operation of the engine are assured.
In a further feature of the invention, it is provided that the high-pressure region includes a common rail and a pressure regulating valve, and that the pressure regulating valve regulates the pressure in the common rail by diverting fuel out of the common rail into the return line, so that the common rail can be scavenged as well.
In a further feature of the invention, it is provided that at least one temperature sensor is present, to ascertain the turn-on criterion or criteria or the turn-off criterion or criteria, so that the pressure elevation upon hot starting is effected only if there is a risk of vapor bubble formation, and on the other hand, the pressure elevation is not maintained any longer than necessary.